With the widespread use and development of the Internet, a large amount of information has been exchanged over communication networks, and thus, there has been a need to transmit and receive a larger amount of information at higher speed. Optical fibers are typically used for such transmission and reception of information. In particular, single-mode fibers made of quartz glass are suitable for high-capacity transmission of information, and are used in great quantities as communications fibers.
Normal single-mode fibers each have a structure in which a central portion of the single-mode fiber includes a core having a high refractive index and surrounded by a cladding with a low refractive index, and are fibers in which only a fundamental mode propagates through the core. While such a single-mode fiber is routed, as a main fiber from an information relay point to a user (e.g., a business office or a home), for example, by being disposed along a power transmission line, other optical fibers are used for routing into buildings and wiring in relay devices, and such other fibers and the main fiber are connected together through connectors, etc. In this case, an optical fiber which has a different structure from the main fiber and of which the bend resistance is enhanced can be used as an optical fiber for use in routing into buildings and wiring in relay devices. The reason for this is that there is a need to route optical fibers in small spaces in buildings or relay devices.